Water management is an important aspect for successful agricultural plant production since water stress may a pronounced effect on the vegetative growth, and number and size and quality of fruits. Excessive irrigation is expensive, can cause vigorous vegetative growth as well as delayed ripening and increases the need for disease and pest control. Excessive water supply will also cause percolation of water below the root zone, leaching nitrate and other chemicals into groundwater. Furthermore, the need to optimize crop water use has become more important given the decrease in the amount of available water for agriculture and the increase in the irregularity of rain distribution.
In practice, various irrigation methods have been developed for optimizing the amount of water and frequency of application in dependency on the evapotranspirative demand, weather conditions and the type of soil. Efficient irrigation requires precise information on the specific water requirements of the plant species at different growth stages and under varying (micro-)climatic conditions, and on crop water consumption to meet the correct irrigation scheduling timing and amount of water, fertilizers and to limit losses through deep percolation.
The water status of leaves can be determined using a pressure chamber (P. F. Scholander et al. in “Science” vol. 148, 1965, p. 339-346). The method is simple, but massively invasive, time-consuming and unsuitable for automation. Further drawback is that the number of leaves that can be measured is rather limited and, therefore, data can be misrepresentative of the overall in-situ conditions (due to variability in height, sun exposure, microclimate conditions, canopy circumference etc.). Most importantly and frequently ignored, the readings cannot always straightforwardly be interpreted in terms of xylem pressure and/or turgor pressure.
Further techniques for investigating the water status of plants, in particular for irrigation purposes, are known in practice. As an example, humidity sensors can be used for measuring soil moisture content directly. Although such sensors can be permanently installed at representative sites in an agricultural field, particular disadvantages are given in terms of soil heterogeneity and the requirement of a close contact to the soil matrix.
The most reliable information on the water status can be obtained if the turgor pressure in the plant cells is measured directly. Various types of turgor pressure measuring devices have been described in the past. An early example is the pressure measurement in plant cells described by U. Zimmermann et al. (“Die Naturwissenschaften”, 1969, vol. 56, p. 634), whereas the turgor pressure is sensed with a combination of a micro-needle and a pressure probe. Further turgor pressure measuring techniques, like e.g. ball tonometry, micro-indentation, cantilever bending measurements or aspiration measurements have been described by U. Zimmermann et al. (“New Phytologist”, vol. 162, 2004, p. 575-615) and by A. Geitmann (“American Journal of Botany”, vol. 93, 2006, p. 1380-1390). As another approach, the relationship between leave thickness and plant water potential has been described by T. McBurney (“Journal of Experimental Botany”, vol. 43, 1992, p. 327-335).
Generally, the conventional turgor pressure measurement techniques have disadvantages in terms of limited reliability and applicability under practical conditions of agriculture. It has been found that the results obtained with the conventional techniques have a limited significance. Furthermore, the conventional techniques are not suitable for long-term outdoor applications especially because of their susceptibility to gusty or high winds.